Re: [PATCH v2 4/5] mm: FLEXIBLE_THP for improved performance

[Ryan Roberts]

On 10/07/2023 10:18, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
> 
>> On 10/07/2023 04:03, Huang, Ying wrote:
>>> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
>>>
>>>> On 07/07/2023 15:07, David Hildenbrand wrote:
>>>>> On 07.07.23 15:57, Matthew Wilcox wrote:
>>>>>> On Fri, Jul 07, 2023 at 01:29:02PM +0200, David Hildenbrand wrote:
>>>>>>> On 07.07.23 11:52, Ryan Roberts wrote:
>>>>>>>> On 07/07/2023 09:01, Huang, Ying wrote:
>>>>>>>>> Although we can use smaller page order for FLEXIBLE_THP, it's hard to
>>>>>>>>> avoid internal fragmentation completely.  So, I think that finally we
>>>>>>>>> will need to provide a mechanism for the users to opt out, e.g.,
>>>>>>>>> something like "always madvise never" via
>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled.  I'm not sure whether it's
>>>>>>>>> a good idea to reuse the existing interface of THP.
>>>>>>>>
>>>>>>>> I wouldn't want to tie this to the existing interface, simply because that
>>>>>>>> implies that we would want to follow the "always" and "madvise" advice too;
>>>>>>>> That
>>>>>>>> means that on a thp=madvise system (which is certainly the case for android and
>>>>>>>> other client systems) we would have to disable large anon folios for VMAs that
>>>>>>>> haven't explicitly opted in. That breaks the intention that this should be an
>>>>>>>> invisible performance boost. I think it's important to set the policy for
>>>>>>>> use of
>>>>>>>
>>>>>>> It will never ever be a completely invisible performance boost, just like
>>>>>>> ordinary THP.
>>>>>>>
>>>>>>> Using the exact same existing toggle is the right thing to do. If someone
>>>>>>> specify "never" or "madvise", then do exactly that.
>>>>>>>
>>>>>>> It might make sense to have more modes or additional toggles, but
>>>>>>> "madvise=never" means no memory waste.
>>>>>>
>>>>>> I hate the existing mechanisms.  They are an abdication of our
>>>>>> responsibility, and an attempt to blame the user (be it the sysadmin
>>>>>> or the programmer) of our code for using it wrongly.  We should not
>>>>>> replicate this mistake.
>>>>>
>>>>> I don't agree regarding the programmer responsibility. In some cases the
>>>>> programmer really doesn't want to get more memory populated than requested --
>>>>> and knows exactly why setting MADV_NOHUGEPAGE is the right thing to do.
>>>>>
>>>>> Regarding the madvise=never/madvise/always (sys admin decision), memory waste
>>>>> (and nailing down bugs or working around them in customer setups) have been very
>>>>> good reasons to let the admin have a word.
>>>>>
>>>>>>
>>>>>> Our code should be auto-tuning.  I posted a long, detailed outline here:
>>>>>> https://lore.kernel.org/linux-mm/Y%2FU8bQd15aUO97vS@xxxxxxxxxxxxxxxxxxxx/
>>>>>>
>>>>>
>>>>> Well, "auto-tuning" also should be perfect for everybody, but once reality
>>>>> strikes you know it isn't.
>>>>>
>>>>> If people don't feel like using THP, let them have a word. The "madvise" config
>>>>> option is probably more controversial. But the "always vs. never" absolutely
>>>>> makes sense to me.
>>>>>
>>>>>>> I remember I raised it already in the past, but you *absolutely* have to
>>>>>>> respect the MADV_NOHUGEPAGE flag. There is user space out there (for
>>>>>>> example, userfaultfd) that doesn't want the kernel to populate any
>>>>>>> additional page tables. So if you have to respect that already, then also
>>>>>>> respect MADV_HUGEPAGE, simple.
>>>>>>
>>>>>> Possibly having uffd enabled on a VMA should disable using large folios,
>>>>>
>>>>> There are cases where we enable uffd *after* already touching memory (postcopy
>>>>> live migration in QEMU being the famous example). That doesn't fly.
>>>>>
>>>>>> I can get behind that.  But the notion that userspace knows what it's
>>>>>> doing ... hahaha.  Just ignore the madvise flags.  Userspace doesn't
>>>>>> know what it's doing.
>>>>>
>>>>> If user space sets MADV_NOHUGEPAGE, it exactly knows what it is doing ... in
>>>>> some cases. And these include cases I care about messing with sparse VM memory :)
>>>>>
>>>>> I have strong opinions against populating more than required when user space set
>>>>> MADV_NOHUGEPAGE.
>>>>
>>>> I can see your point about honouring MADV_NOHUGEPAGE, so think that it is
>>>> reasonable to fallback to allocating an order-0 page in a VMA that has it set.
>>>> The app has gone out of its way to explicitly set it, after all.
>>>>
>>>> I think the correct behaviour for the global thp controls (cmdline and sysfs)
>>>> are less obvious though. I could get on board with disabling large anon folios
>>>> globally when thp="never". But for other situations, I would prefer to keep
>>>> large anon folios enabled (treat "madvise" as "always"),
>>>
>>> If we have some mechanism to auto-tune the large folios usage, for
>>> example, detect the internal fragmentation and split the large folio,
>>> then we can use thp="always" as default configuration.  If my memory
>>> were correct, this is what Johannes and Alexander is working on.
>>
>> Could you point me to that work? I'd like to understand what the mechanism is.
>> The other half of my work aims to use arm64's pte "contiguous bit" to tell the
>> HW that a span of PTEs share the same mapping and is therefore coalesced into a
>> single TLB entry. The side effect of this, however, is that we only have a
>> single access and dirty bit for the whole contpte extent. So I'd like to avoid
>> any mechanism that relies on getting access/dirty at the base page granularity
>> for a large folio.
> 
> Please take a look at the THP shrinker patchset,
> 
> https://lore.kernel.org/linux-mm/cover.1667454613.git.alexlzhu@xxxxxx/

Thanks!

> 
>>>
>>>> with the argument that
>>>> their order is much smaller than traditional THP and therefore the internal
>>>> fragmentation is significantly reduced.
>>>
>>> Do you have any data for this?
>>
>> Some; its partly based on intuition that the smaller the allocation unit, the
>> smaller the internal fragmentation. And partly on peak memory usage data I've
>> collected for the benchmarks I'm running, comparing baseline-4k kernel with
>> baseline-16k and baseline-64 kernels along with a 4k kernel that supports large
>> anon folios (I appreciate that's not exactly what we are talking about here, and
>> it's not exactly an extensive set of results!):
>>
>>
>> Kernel Compliation with 8 Jobs:
>> | kernel        |   peak |
>> |:--------------|-------:|
>> | baseline-4k   |   0.0% |
>> | anonfolio     |   0.1% |
>> | baseline-16k  |   6.3% |
>> | baseline-64k  |  28.1% |
>>
>>
>> Kernel Compliation with 80 Jobs:
>> | kernel        |   peak |
>> |:--------------|-------:|
>> | baseline-4k   |   0.0% |
>> | anonfolio     |   1.7% |
>> | baseline-16k  |   2.6% |
>> | baseline-64k  |  12.3% |
>>
> 
> Why is anonfolio better than baseline-64k if you always allocate 64k
> anonymous folio?  Because page cache uses 64k in baseline-64k?

No, because the VMA boundaries are aligned to 4K and not 64K. Large Anon Folios
only allocates a 64K folio if it does not breach the bounds of the VMA (and if
it doesn't overlap other allocated PTEs).

> 
> We may need to test some workloads with sparse access patterns too.

Yes, I agree if you have a workload with a pathalogical memory access pattern
where it writes to addresses with a stride of 64K, all contained in a single
VMA, then you will end up allocating 16x the memory. This is obviously an
unrealistic extreme though.

> 
> Best Regards,
> Huang, Ying
> 
>>>
>>>> I really don't want to end up with user
>>>> space ever having to opt-in (with MADV_HUGEPAGE) to see the benefits of large
>>>> anon folios.
>>>>
>>>> I still feel that it would be better for the thp and large anon folio controls
>>>> to be independent though - what's the argument for tying them together?
>>>>
>